This invention relates generally to telephone switching networks.
As is known in the art, narrowband metropolitan and regional public telephone switching networks are typically based on the interconnection of local end office switches via tandems in a hierarchical arrangement, or when required by the volume of traffic, via direct trunk groups. National telephone switching networks usually use a mesh topology to interconnect regionally located tandems in a non-hierarchal, or mesh switching network. One such mesh switching network suggested is an ATM mesh switching network used with a Synchronous Optical Network (SONET) ring, as shown in FIG. 1. Here, for example, nodes A through E serve a plurality of narrowband (NB) switches, the NB switches at each node being here arranged in a hierarchial architecture. Each hierarchial structure of NB switches terminates in an ATM switch at the node, as shown. It is noted that with N nodes, there are N(N−1)/2 paths which are possible between the nodes. For example, with the 5 node arrangement shown in FIG. 1, there are 10 possible paths between the various nodes (i.e., paths between nodes A and B, A and C, A and D, A and D, A and E, B and C, B and D, B and E, C and D, and D and E). It is noted that there may be up to 16 nodes on the ring, resulting in 120 paths.
Each ATM switch is coupled to the SONET ring through an add-drop multiplexer (ADM). Each ADM takes traffic on the ring and drops it off to the ATM switch coupled thereto (i.e., its associated ATM switch) or takes traffic from the associated ATM switch and couples it onto the ring.
The total capacity of the SONET ring is fixed, for example, 2.5 gigabits/second (Gb/s). The total ring capacity must be distributed among the paths, here among the 10 paths. This distribution is established by traffic engineering (i.e., estimating the expected traffic distribution among the paths) and once established, implemented by pre-setting, or pre-conditioning, the ADMs. For example, if the ADM at node B were pre-set at 480 channels between nodes A and B and all 480 channels are in use, a new call being made at node A for node B, would not be able to be routed directly from node A to node B. Rather, assuming in this example that the capacity between nodes A and C where not totally used and the capacity between channel B and C were not totally used, the new call may be routed from the ATM switch and the ADM at node A, then to the ADM at node B, then to the ADM and ATM switch at node C, and finally to the ADM and ATM switch at node B. Thus, the traffic flows on the ring twice, i.e., between nodes B and C and then from Node C to node B, thereby reducing the efficiency switching network, as indicated by the dotted arrow in FIG. 1.
As is also known in the art, an ADM can operate in the Time Division Multiplexing (TDM) mode or the ATM mode. While ADMs today operate in a TDM mode, the use of an ATM ADM has been suggested to increase SONET utilization efficiency by using virtual paths as the traffic entities which are added or dropped to or from the SONET ring instead of using fixed bandwidth virtual tributaries. With such suggested arrangement, a virtual path may have any bandwidth up to the SONET ring line rate limit. Such an arrangement is described in Bellcore GR-2837-CORE Issue 2, “ATM Virtual Path Functionality in SONET Rings” published December 1995, the entire subject matter thereof being incorporated herein by reference.
As is also known in the art, two distinct methods can be used for interworking narrowband traffic into an ATM network: network interworking, which is also known as landline interworking; and, service interworking. Service interworking envisions the ATM network as a full-service B-ISDN while network interworking simply uses the ATM network as a more bandwidth-efficient transport network. Service interworking involves mapping of narrowband services and signalling into a native ATM environment (e.g., B-ISUP signalling). Network interworking, as used herein, provides transparent transport of narrowband services and signaling between narrowband switches through an ATM network.